External hard drives between Mac and PC

I recently purchased a seagate external hard drive and i would like to use it with my mac and pc. I connected to my pc and added files to it. then when i connected it to my mac the files were able to be opened but when i went to add files to the hard drive from the mac it told me that the hard drive couldnt be modified so i formatted it for a mac and then when i went to look at the files on my Pc it didint recognize the hard drive. Any advice in how i can make it so i can use my hard drive between both my mac and pc and be able to add files from both my mac and pc and be able to open them on both?

since you connected it to a pc first its formatted for windows.
now for mac you can copy what ever you want out of it. use it for a daw for mixing etc. but you can save anything back to it.
i did the same thing you did too lol.
theres certain term for it one of these smart guys arounf here will post more info im sure.
you have to get another drve for your mac if you want to backup stuff or reformat it for the mac.

Backup everything on the external to your computer (a Mac or your PC).
Reconnect your external.
Use Disk Utility (in your Applications/Utilities folder of OS X) to create two partitions on your external.
One partition should be Apple's file system, Mac OS X Extended journaled (aka HFS+).
The other partition should be a Windows filesystem, either NTFS or FAT32, depending on what you want to be able to do with those files.
Tell Disk Utility to create the partitions. You can resize them if you want.

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